My understanding of his position is that he does not consider firmware to be independent software. The arguments by which he came to be in favor of free software do not apply to firmware.
The reason that he promotes free software is because software is capable of being free, that is, it is logistically realistic that people freely copy, modify, and distribute it. From this, he extrapolates that there is a right to copy, modify, and distribute software. If would be physically capable of modifying the code, but is legally prevented, then that is a restriction on his rights.
Under his reasoning, it also does not apply to firmware. Because it is so tied to the underlying hardware, it's not really possible to copy or modify it in any useful way. It is certainly free, in the monetary sense, but there are no useful results of doing so. Therefore, closed software does not restrict your rights in a meaningful or non-trivial sense.
Note that this is just my interpretation of what I've read of him, and I don't follow the Free Software thing that closely so I could be mis-interpreting key points.